A torsional damper (TVD) is attached to a crankshaft for the purpose of reduction of engine vibration (reduction of torsional vibration of the crankshaft).
For example, the torsional damper is configured such that a hub (a vibrating body) attached to a shaft end of the crankshaft is coupled to an annular mass member (damper wheel) disposed on the outer circumference thereof, via an elastic body made of rubber.
Positive and negative accelerations are alternately generated in the crankshaft in accordance with the drive of an engine, resulting in a natural frequency. On the other hand, the mass member disposed on the crankshaft attempts to continue rotating at a constant rotation speed. Therefore, the interposed torsional damper is required to have a function of absorbing a change in angular speed therebetween. Thus, a main function of the torsional damper is to match a torsional-direction natural frequency of the torsional damper to a natural frequency generated in the crankshaft to reduce the torsional vibration of the crankshaft for preventing breakage of the crankshaft and reducing noise. The natural frequency of the torsional damper is determined depending on a spring constant of rubber and inertial mass of a vibration ring.
To tune the natural frequency of the torsional damper, a rubber material used for the torsional damper is required to have hardness variation (type A durometer) on the order of Hs 50 to 80°.
To reduce a change in natural frequency due to working temperature and enable absorption of vibration in a wider temperature range, the rubber material is required to have favorable heat resistance of a rubber elastic body and favorable temperature dependence of a spring constant (favorable temperature characteristic of vibration characteristic (E′)).
Although the tuning is conventionally performed by using the spring constant at 60° C. to absorb vibration in a normal working temperature range that is a temperature range of about 20 to 100° C., a working range of the torsional damper exceeds 100° C. on the high temperature side and, therefore, favorable hot physical properties at high temperature are also required.
A current mainstream torsional damper is of a fitting type fabricated by press-fitting rubber molded by vulcanizing molding into a ring shape between the hub attached to the crankshaft and the vibration ring. Since the vibration ring is prevented from slipping by compression stress of this fitting torsional damper, degrading of repulsive stress associated with fitting compression must be small over time.
Although conventional EPDM material has well-balanced physical properties in terms of the hardness variation, the temperature dependence of the spring constant, the hot physical properties, and the repulsive stress at the time of fitting, the EPDM material has smaller tan δ in vibration characteristic as compared to NBR material, ACM material, AEM material, and IIR material and tends to have smaller vibration reduction effect. Therefore, it is desirable to develop EPDM material having a high damping performance.
For this goal, attempts have been made to realize desired physical properties and a high damping performance by changing formulation of compositions.
For example, Patent Document 1 proposes to use for a damper a crosslinked substance of an EPDM composition made of (a) 100 parts by weight of EPDM that is at least one type of EPDM having a propylene content (C3/(C2+C3)) of 35 to 50 wt % in the total amount of ethylene and propylene in the copolymer rubber and Mooney viscosity (ML100) of 40 or more, (b) 5 to 50 parts by weight of a-olefin oligomer that is polymer of an α-olefin represented by a general formula CH2=CHR (where R is an alkyl group having a carbon number of 3 to 12) and that has a number average molecular weight Mn of 300 to 1,400, and (c) 1 to 10 parts by weight of an organic peroxide crosslinking agent. The crosslinked substance of the EPDM composition produces effects of improving the temperature dependence of the spring constant in a low temperature range, achieving a favorable damping performance at normal working temperature, suppressing vibration amplification at a resonance point, and achieving favorable durability.
With regard to physical properties of an EPDM composition, Patent Document 2 proposes a rubber composition for heat-resistant conveyor belts acquired by blending 100 parts by weight of ethylene-propylene rubber with 5 to 25 parts by weight of ethylene-α-olefin oil and describes that the abrasion resistance and the cracking resistance of the EPDM composition can be improved by using ethylene-α-olefin oil as a softener.
In Patent Document 3, low hardness (equal to or less than JIS-A of 10) and high damping performance are attained by an EPDM composition that is made of 100 parts by weight of oil-extended EPDM (as EPDM), 20-150 parts by weight of ethylene-α-olefin copolymer having a number average molecular weight Mn of 10,000 or less, and 10 to 150 parts by weight of an ester plasticizer and that is crosslinked (vulcanized) with a sulfur-based vulcanizing agent.
However, if the high damping performance is achieved by simply changing formulation, since well-balanced physical properties of conventional material are significantly degraded and original features of EPDM material are damaged, a problem still remains in that not all the required physical properties are satisfied.